1. Field of the Invention
The present invention relates to a polysulfide based resin lens, comprising a mixture of at least one epoxy and/or episulfide resin (hereinafter referred to as epoxy resin) having at least two epoxy groups and/or episulfide groups, and at least one polythiol compound having at least two mercapto groups and at least one internal releasing agent, which is cast, polymerized, and the process for preparing the same. Plastic lenses are lightweight, less fragile than inorganic lenses and can be dyed with ease.
2. Description of the Prior Art
A resin currently widely used in lenses is a radical polymerization product of diethylene glycol bisallyl carbonate (hereinafter referred to as DAC). The resin has excellent impact resistance, is lightweight, has prominent dye-affinity and good machinability including cutting ability and polishing ability.
However, lenses prepared from the resin have a smaller refractive index (n.sub.D =1.50) than inorganic lenses (n.sub.D =1.52). In order to obtain equivalent optical properties as glass lenses, it is necessary to increase the center thickness, peripheral thickness and curvature of the lens and hence the lens as a whole becomes inevitably thick. Therefore, resins providing a higher refractive index are desired.
Other resins which have been proposed to provide high refractive indices include resins obtained by reacting an epoxy resin with acrylic acid or methacrylic acid and subsequently radical polymerizing the resultant epoxy (meth)acrylate compound (Japanese Pat. Laid-Open No. 164607/1983), resins obtained by curing an epoxy resin with acid anhydride (Japanese Pat. Laid-Open No. 164618/1983 and 22001/1984), resins obtained by curing an epoxy resin with acid anhydride in admixture with a vinyl monomer (Japanese Pat. Laid-Open No. 164617/1983), resins obtained by curing an epoxy resin with an aromatic compound having phenolic hydroxyl groups (Japanese Pat. Laid-Open No. 24201/1988); and resins obtained by copolymerizing epoxy resins with each other (Japanese Pat. Laid-Open No. 93720/1984).
No prior art, however, has been known on the preparation of lens by curing epoxy resins with polythiol compounds.
On the other hand, plastic lenses are a rigid body completely free from bubbles that is required to have a high surface accuracy, such an optical homogeneity that any strain cannot be found by an optical strain meter, and high transparency. Generally, as a method of making plastic lenses, use is made of cutting and abrading, hot press molding, cast molding, injection molding or the like, and when a thermosetting resin such as DAC is used as a material, cast molding is used. In this case, the releasability between the lens and the mold is important, and if the release is premature, the lens surface is disturbed, or the lens is liable to be strained, whereas if the release becomes difficult, exfoliation of the mold takes place. Although it is known that butyl phosphate is used as a releasing agent for DAC, generally it is not preferable to use an internal releasing agent in view of the physical properties of the lens (see "Polymer Digest" by Seiichi Miuma, 3, 39 (1984), etc.).
However, generally when an epoxy resin based lens is cast, the adhesion between the lens and the mold is so good that the lens cannot be released from the mold easily.
However, although lenses prepared from the preceding resins have higher refractive indices than lenses prepared from DAC, the lenses do not exhibit the desired high refractive indices. Further, since such resins are prepared from compounds containing many halogen atoms or aromatic rings to improve the refractive index, lenses prepared from these resins have disadvantages such as large dispersion of refractive index, poor weatherability and high specific gravity.
On the other hand, as a method of improving the releasability for casting epoxy resin based lenses, we have tried a method of treating the mold with an external releasing agent and a method that uses a mold of a polyolefin resin.
However, the method wherein a fluorine type external releasing agent or a silicon type external agent is used to form a release film on the mold was accompanied by such problems that the thickness of the release film was difficult to become constant, part or all of the release film was transferred to the surface of the polymerized lens or into the inside of the polymerized lens which caused deterioration of the surface, and more specifically the surface was made non-uniform or the lens became turbid. In addition, because it was required to treat the mold with a releasing agent every time when the mold was used repeatedly, it was found that the method was troublesome as an industrial lens molding method, the productivity of the lens by the method was low, and the method was quite uneconomical.
On the other hand, in the method that uses a mold of a polyolefin resin, because a high temperature deforms the resin mold, the surface accuracy of the molded lens was poor, and it was found that, in the field of optical lenses and lenses for glasses wherein a high surface accuracy is required, an improvement in the method was required.